The present invention relates to a magneto-optical medium suitable for magneto-optical recording, etc., and more in detail, relates to a magneto-optical medium comprising a magnetic layer, a protective layer of a complex oxide containing aluminium oxide and a base plate.
Among the optical memory elements, as an erasable type memory which can add or erase the record, a magneto-optical recording system is highly practicable.
As the medium for the magneto-optical recording system, an alloy thin film comprising rare earth element(s) and transition metal(s) is most excellent from the view point of the all-around memory properties. However, an alloy thin film comprising rare earth element(s) and transition metal(s), has a fatal defect in that such a thin film is poor in corrosion-resistance.
Namely, since such an alloy thin film comprising rare earth element(s) and transition metal(s) is poor in corrosion-resistance, the film causes, accompanying with the progress of corrosion thereof, the reduction of magnetic cohesive force which is the necessary condition of the high density recording, the reduction of the Kerr rotation angle which is the necessary condition of the high S/N ratio and the increase of the error rate.
Heretofore, in order to improve the corrosion-resistance of the film, the following two methods have been adopted:
(1) to improve the corrosion-resistance by adding an additive agent to magnetic layer and
(2) to form a protective layer for improving the corrosion-resistance.
Of those two methods, in the method of using a protective layer for improving the corrosion-resistance, at first, a protective layer of an oxide of high melting point such as Al.sub.2 O.sub.3, etc. has been proposed, because these oxides are low in reactivity to Tb, Fe, etc., and they are low in reactivity at the interface.
Namely, since the reactivity of the interface between the magnetic layer and the protective layer is low (Generally, an oxygen emited from the oxide often reacts with Tb etc., however, in the case where the oxide is Al.sub.2 O.sub.3, such a reaction is substantially reduced since a bond between aluminum and oxygen is strong.), the deterioration of the magnetic layer is low and accordingly the protective layer of the oxide of high melting point is preferable.
However these oxides have a disadvantage of forming cracks, in the case of forming a layer on a plastic base plate. In addition, these oxides have faults that they are low in anti-moisture property resulting in forming smear-like spots on their surface exposed to a wet atmosphere under a high humidity and making the incidence of the read/write laser beam thereinto difficult, and as the results, the optical characteristic are degraded.
Furthermore, it has been elucidated that as the protective layer other than the above-mentioned oxide, since the layer of nitrides such as AlN and Si.sub.3 N.sub.4 and the layer of fluorides such as MgF.sub.2, CeF.sub.3, etc. do not contain oxygen, such a layer has a favorable protecting characteristic. However, for these protective layers problems have been pointed out that (1) the layer of nitrides is high in thermal conductivity and the leakage of heat is large at the time of laser irradiation to the recording medium, (2) the adhesion force between the nitride layer and the magnetic layer is weak, and (3) the floride layer is difficult to form the film structure dense enough to protect the penetration of the oxygen and/or water.
Also, in the magneto-optical medium comprising the base plate such as glass, metal, plastic etc. and magnetic layer, in the case of using the plastic such as acrylic resin, polycarbonate resin, etc. as the base plate, it has been pointed out that (1) an affinity of the base plate and the magnetic layer is inferior, and as a results the cracks are formed and/or the exfoliation occurs, and (2) the deterioration of the magnetic layer occurs by a small amount of air, water etc. which are contained in the base plate or are penetrated through the base plate. Accordingly, the protective layer is required that (1) the affinity of the protective layer and the base plate or the magnetic layer is high, (2) water, air etc. are not penetrated, and (3) the protective layer per se is not deteriorated by water, air etc.
Further, in the case where the protective layer is placed between the base plate and the magnetic layer (the protective layer is placed on the surface of the magnetic layer, of which an incident bean are irradiated), it is desirable that the transparency of the protective layer is necessary so far as the incident beam reach enough the magnetic layer and a material of the protective layer has a high refractive index in order to improve the sensitivity of the magneto-optical medium at reading.
Accordingly, as a result of the present inventors' studies on the protective layer with the objective of improving the above-mentioned demerits of the hitherto proposed protective layers while paying their attention to the non-reactivity of Al.sub.2 O.sub.3 at the interface, it is found that a protective layer comprising a complex oxide containing aluminum oxide solves the above-mentioned problems, and based on the finding, the present invention has been attained.
Namely, the objective of the present invention is to provide a protective layer which has the non-reactivity with the magnetic layer at the interface, an excellent affinity to the base plate of plastic such as acrylic resin, polycarbonate resin, etc., a low heat-conductivity and a high refractive index, and which is not deteriorated by water, air, etc. and is readily formed.